New port: devel/llbuild

llbuild is a set of libraries for building build systems. Unlike most
build system projects which focus on the syntax for describing the
build, llbuild is designed around a reusable, flexible, and scalable
general purpose build engine capable of solving many "build system"-like
problems. The project also includes additional libraries on top of that
engine which provide support for constructing bespoke build systems
(like swift build) or for building from Ninja manifests.

WWW: https://github.com/apple/switch-llbuild

Approved by: mat
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8439